Tiny polyps save corals from predators and disease

In a new study published today in Proceedings of the Royal Society, scientists show how tiny hydrozoans, polyps smaller than one millimeter and commonly found in dense colonies on the surface of hard corals, may play a role in keeping corals safe and healthy.

Tibor Navracsics, Commissioner for Education, Culture, Youth and Sports, responsible for the Joint Research Centre (JRC) said: "EU scientists are actively contributing to solve global problems. This is a good example of the scientific excellence that can help us to protect our environment and support the policy making."

Tiny polyps protect corals from being eaten by fish and snails

The researchers from the JRC, Marhe Center of Milan-Bicocca University and the King Abdullah University of Science and Technology researched almost 2500 coral colonies in Maldivian and Saudi Arabian reefs, searching for signs of predation, temperature induced stress, and disease. For each colony, they also looked for the presence or absence of symbiotic hydrozoans and concluded that corals living in association with hydrozoans are much less prone to be attacked by coral-eating fish and gastropods, such as snails, than hydrozoan-free corals.

Predators are likely deterred by the venomous organelle ejected by hydrozoans' nematocysts, similar to those found in medusas' tentacles. Obviously, an individual hydrozoan polyp of less than one millimeter cannot cope with fish with a body mass billions of times its own. Yet when hydrozoans reach very high densities on coral surface, sometimes more than 50 individuals per square centimeter, it creates a sort of a continuous, stinging carpet that can discourage fish from foraging.

This effect is even more pronounced for coral eating gastropods, which are exposed to hydrozoans' nematocysts longer and more intensely as they crawl. This finding is important for coral conservation because coral eating gastropods are regarded as a major threat to reef persistence as a consequence of warming water temperatures.

The authors also found that the presence of symbiotic hydrozoans goes hand in hand with better health of corals. This could be either a secondary effect as wounds caused by predators are more easily infected by pathogens, and/or the result of "friendly" hydrozoans feeding on pathogenic protozoans trying to invade coral hosts.

Unfortunately, even hydrozoans cannot protect corals against mortality directly associated to increasing temperature (i.e. coral bleaching), as demonstrated by the massive coral die out recorded last year in the Great Barrier Reef. Yet, by contributing to the overall health of corals, they are likely playing an important role in making reefs more resilient, possibly shifting the critical threshold between reversible and irreversible disturbances. If this is the case, this would mean that an almost invisible and largely overlooked microscopic organism could be buying time for half a billion people relying on coral reef ecosystems for their livelihoods.

Researchers from The University of Western Australia (UWA), ARC Centre of Excellence for Coral Reef Studies, and Western Australian Marine Science Institution have examined the impact of the 2016 mass bleaching event on reefs ...

Greenland is melting faster than scientists previously thought—and will likely lead to faster sea level rise—thanks to the continued, accelerating warming of the Earth's atmosphere, a new study has found.

For many years, China has been struggling to tackle high pollution levels that are crippling its major cities. Indeed, a recent study by researchers at Chinese Hong Kong University has found that air pollution in the country ...

A two to three-fold increase in heatwave activity in the United Kingdom since the late 19th century has been identified in a new analysis of historical daily temperature data led by University of Warwick scientists.

An international team of researchers has found evidence that suggests the cooling effect of aerosols in cumulus and MSC clouds is twice as high as thought. In their paper published in the journal Science, the group describes ...

0 comments

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.